The main function of an intake manifold is to distribute clean air—as far as possible, without pressure loss—from the air filter into the combustion chamber of the engine. As well as the throttle body, further components such as an air mass meter, sensors, EGR ducts, fuel rail and injection valves may be integrated into the complete air intake system.
A typical intake manifold may be mounted to a cylinder head of an engine includes a plenum and may include a one runner for each engine cylinder that distributes air flow from the manifold to the intake ports of each cylinder. For a given air intake manifold, engine performance (e.g., the location of the engine's torque peak in the RPM band) is a function of the volume of the plenum, the cross-sectional area of the runners and, to a lesser extent, the length of the runners.
Conventional intake manifolds employed on engines generally have fixed runner geometry. With a fixed intake system, the speed at which intake tuning occurs is also fixed. Since the engine operates over a broad RPM range, and since a different geometry may be ideal for different engine speeds, fixed geometry intake systems are designed with a geometry which is only optimal for a limited engine speed range, thus the engineer is forced to design a compromise between torque at low speeds and horsepower at high engine speeds.
In a tuned manifold, for example, the plenum volume, the length of the runners and the cross-sectional area of the runners may be selected so that a pressure wave formed within the runners has a resonant frequency that optimizes (or elevates) the intake pressure at each intake port when the corresponding intake valve opens, providing increased mass air flow to the cylinder.
One variable used to select the size and dimension of both the plenum and the runners is the engine size (i.e. the engine displacement). The total volume of an air intake manifold, which includes the volume of the plenum and the volume within the runners, is typically about twice the total engine displacement.
The performance and torque of internal combustion engines are significantly improved through the development of variable air intake manifolds which can switch between different runner lengths. A short runner optimizes performance at higher speeds, while a long runner provides favorable torque in the lower and medium speed ranges.
An active air intake manifold optimizes incoming airflow through a valve provided in the intake manifold. The valve controls flow to the intake tracts that correspond to desired engine-performance parameters. At low engine rotational speeds (RPMs), the valve creates a longer path for intake air, enhancing combustion efficiency and torque output. At higher engine rotational speeds, the valve opens, creating a shorter path for maximum engine power.
Active air intakes are particularly useful in adapting the intake manifold to significantly increase low speed engine torque, giving the engine a broader torque curve that retains higher specific torque output across the engine speed range.
Some intake system designs have been created to allow for variable intake geometry and have met with varying degrees of success. With these designs, the cost may be excessive for certain applications due to complex and costly design, either in fabrication or assembly. An example of a suitable but relatively more costly solution is disclosed in European Patent publication EP1135584.
Other lower cost designs may utilize slide valves or air flaps. Either type is typically driven by a vacuum actuator or an electric motor, usually under the control of an Engine Control Unit (ECU) computer. Designs of this type close and open only the short runner, closing the short runner at low speeds to improve engine low speed torque.